wij ontvingen een attest van uw bevallingsrust in nederland van 20.08.2007 tem 28.08.2007. dit is onvolledig,wij hebben ook een attest nodig van 19.05.2007tem 19.08.2007
I think 'bevallingsust' should be 'bevallingsrust'.
An 'attest' is a written evidence, certificate. It exists in English too, but I'm not sure if it can be used as a noun.
'Bevallingsrust' is a period of rest after a mother has given birth in which she doesn't have to work.
I'm not sure how I should translate those words in English, maybe Chantal knows or maybe these descriptions are sufficient for you. The rest of the sentence would be:
We received an 'attest' of your 'bevallingsrust' in the Netherlands from 20.08.2007 to 28.08.2007. This is incomplete, we also need an 'attest' from 19.05.2007 to 19.08.2007.